1. Field of the Invention
The present invention relates to an in-band group delay equalizer and a distortion compensation amplifier for use in a high-frequency band.
2. Description of the Related Art
In the art of base stations for mobile radio communication systems, a large number of distortion compensation amplifiers are used for the purpose of reducing the size of base stations.
A known technique to realize a distortion compensation amplifier is to use a feedforward amplifier. In this technique, it is required that the group delay time of a high-power path and that of a low-power path should be equal to each other in both a distortion detection circuit and a distortion suppression circuit. To obtain equal group delay times, coaxial cables were used in the early days of the technology. In recent years, reductions in size and loss have been achieved by using delay filters instead of coaxial cables.
In delay filters used for this purpose, it is required that the group delay characteristic thereof should be flat over a passband (variation in group delay time within the passband should be small). Conventionally, the delay filter is formed of a multi-stage bandpass filter. FIGS. 14 to 16 show an example of a delay filter formed of a multistage bandpass filter. FIG. 14 shows an equivalent circuit of a delay filter including eight resonators. In FIG. 14, reference symbols Ra to Rh denote resonators. Adjacent resonators are coupled with each other via a capacitor.
FIG. 15 shows the structure of the delay filter. In FIG. 15, reference numeral 4 denotes a substrate. Coaxial resonators Ra to Rh and a coupling board 21 on which a plurality of capacitors are formed, are disposed on the upper surface of the substrate 4. The central conductor of each coaxial resonator is connected to one of electrodes formed on the coupling board 21.
FIG. 16A shows the group delay characteristic of this delay filter, and FIG. 16B shows the transfer characteristic thereof.
Japanese Unexamined Patent Application Publication No. 2001-257505 discloses a delay filter formed by adding a parallel capacitor for jump coupling to a common-type bandpass filter. An example of such a delay filter is shown in FIG. 17 to 18, wherein FIGS. 17 and 18 show an equivalent circuit and the structure thereof, respectively. In this example, the second-stage resonator Rb and the fifth-stage resonator Re are jump-coupled with each other via the parallel capacitor. In FIG. 18, reference numeral 22 denotes a coupling board for realizing the jump coupling. FIGS. 19A and 19B show the group delay characteristic and the transfer characteristic, respectively, of this delay filter.
W001/01511A1 discloses a technique of equalizing the overall group delay characteristic by adding a circuit having a convex group delay characteristic to a bandpass filter having a concave group delay characteristic. FIGS. 20A and 20B show examples of the group delay characteristic and the transfer characteristic, respectively, of the filter disclosed in W001/01511A1. In FIG. 20A, curve b indicates the concave group delay characteristic and curve c indicates the convex group delay characteristic, employed in the this filter, and the overall in-band group delay characteristic obtained by combining them is indicated by curve a. In FIG. 20B, S21 indicates the input-to-output transfer characteristic, and S11 and S22 indicate the reflection characteristics at the input port and the output port, respectively.
In those conventional techniques described above, to achieve good characteristics such as 2100 to 2170 MHz for the passband, 7.5 ns for the group delay, and 0.2 ns for the group delay variation, eight (eight-stage) dielectric resonators are needed in the case of the multi-stage bandpass filter, and six (six-stage) dielectric resonators are needed in the case of the bandpass filter disclosed in Japanese Unexamined Patent Application Publication No. 2001257505 and in the case of the group delay filter disclosed in W001/01511A1.
In any of the conventional techniques described above, the group delay has peaks near both edges of the passband, and thus it is difficult to achieve a flat characteristic in terms of the group delay over a wide band. To increase the group delay bandwidth, the number of resonators of the bandpass filter has to be increased. However, the increase in the number of resonators results in increases in outer dimension and insertion loss. Additionally, the increase in the number of resonators causes a further increase in peaks of the group delay near edges of the passband.
Furthermore, to change the group delay time of the delay filter according to any of the conventional techniques described above, the bandwidth of the bandpass filter must be changed. In general, when the bandwidth of the bandpass filter is changed, the coupling factors between resonators and the resonant frequencies of respective resonators needs to be optimized. Thus, in mass production of delay filters, difficult and time-consuming adjustment is needed.
Furthermore, the capacitance of the parallel capacitor for jump coupling is small compared with the capacitance of capacitors for coupling adjacent resonators, and thus the jump coupling is influenced significantly by stray capacitance which is not shown in the equivalent circuit illustrated in FIG. 17. As a result, produced delay filters have a large variation in characteristic.